tcg/ppc: Change TCG_REG_RA to TCG_REG_TB

At this point the conversion is a wash.  Loading of TB+ofs is
smaller, but the actual return address from exit_tb is larger.
There are a few more insns required to transition between TBs.

But the expectation is that accesses to the constant pool will
on the whole be smaller.

Signed-off-by: Richard Henderson <rth@twiddle.net>